This invention relates primarily to lenses composed of a single material and to systems employing such lenses. More particularly, it relates to designs for such lenses which, when used singly or as part of a system, have the property that certain of their optical characteristics remain relatively unchanged with changes in temperature or change in a controlled predetermined way.
Correction of lens aberrations normally requires the use of multiple component lenses even when thermal effects are not a dominant factor. However, there have been some simple designs reported for use over modest temperature ranges where just portions of a single element lens can correct a specific aberration. For example, chromatic aberrations have been corrected by using a single element lens with a conventional refractive or "bulk" portion, and a shallow surface diffractive portion. Here, the diffractive portion reduces the chromatic aberrations introduced by the bulk portion. (W. C. Sweatt, Applied Optics, Vol. 16, No. 5, May (1977).
However, if thermal effects cannot be ignored because they affect performance due to unacceptable changes due to thermal changes in material or geometric properties of a lens, designs for compensating for them i.e., athermalized designs can of necessity become quite complicated. For example, plastic and glass optical materials may change enough so that corresponding changes in focal length or the state of correction of a lens can become intolerable. This is so in part because these materials, especially plastic exhibit large changes in refractive index with temperature changes.
Designers have made lens systems less sensitive to these temperature effects by exploiting the differences in which changes in refractive index or geometry occur in one or more elements to compensate for those introduced by others. This approach, when properly implemented, can result in a thermally balanced arrangement. For example, if it were important to maintain back focal length in a multiple component lens constant over a given temperature, a designer could adjust the properties of individual elements of the lens in such a way the thermally induced increases to back focal length were balanced or offset by the decreases in others. This could be done by control of the thermal properties of lens element geometry or index, or both.
Such compensation may be important, for example, in arrangements used to focus laser beams onto the surfaces of compact disks.
While such a solution suffices for lenses having multiple components, it is unsuitable for systems using a single lens element made of one material. Hence, there continues to be a need for simple and uncomplicated lens elements or components by which thermal effects can be usefully controlled, and it is a primary object of this invention to provide such.
Another object of the invention is to provide a single-material lens with selected optical characteristics that vary with temperature in a selected manner, for example, in a manner which maintains an optical characteristic substantially constant at a given wavelength.
Another object is to supply an athermalized single-material lens with substantially constant focal length, spherical aberration correction, coma correction, or any combination of these.
Yet another object of the invention is to provide a single-material lens that compensates not only for temperature-induced optical variations of the lens itself but also the temperature-induced variations in the structure spacing the lens from an object or a sensor at or near a specific dominant wavelength.
Other objects and advantages of the invention will become evident from the following detailed description when read in light of the accompanying drawings.